-LRB- CNN -RRB- -- Two Alaskan volcanoes that began minor eruptions weeks ago are showing their strongest seismic activity yet , emitting small amounts of lava and ash along with smoke plumes , the Alaska Volcano Observatory said Tuesday .

The observatory warned of more vigorous activity with the Pavlof and Veniaminof volcanoes , both on the Alaskan Peninsula -- though that will likely just mean more ash .

Pavlof , a snow-covered , cone-shaped mountain , has been erupting since early May . While its activity since then has waxed and waned , seismic activity increased since Tuesday morning and the volcano has started continuously shaking , said David Schneider , a geophysicist with the U.S. Geological Survey .

Some lava is flowing , and the volcano is spewing a cloud of steam and ash that has risen as high as 28,000 feet , said Schneider , at the observatory in Anchorage .

`` It 's a pretty good size , but not high enough to affect overflying aircraft between America and Asia , '' he said . `` It 's more of a problem for local aviation . ''

Pavlof is near the town of Cold Bay , a regional transportation hub whose long World War II-era runway serves flights to area villages , Schneider said . The ash has at times affected those local flights .

Veniaminof , about 60 miles from Pavlof , started showing activity in early June and began erupting June 13 , Schneider said . It has also produced a lava flow and an ash plume , though not as high as Pavlof -- only about 8,200 feet .

Veniaminof is a broad mountain topped with a large , flat ice field , and in the middle is a small cone that produces the lava and ash .

The eruptions of both volcanoes compare to those in the past , where activity lasted for weeks or months before dying down again . The main hazard now , Schneider said , is ash fall , which is more of a nuisance than anything else .

`` So far there 's been trace amounts of ash , so it 's just a fine coating , '' he said . `` Our best estimate of what is going to happen is similar to what 's happened in the past -- this level of activity and some level of ash . ''

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The volcanoes are both on the Alaskan Peninsula

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They began erupting weeks ago but are now showing stronger seismic activity

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They are producing small amounts of lava and ash

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One of them is producing smoke plumes as high as 28,000 feet